Latest geckodriver fails to load an extension on Firefox 68.0

Background

We currently use the geckodriver@3.1.0 version. When trying to update it to the latest version, loading an extension fails:

% docker run --shm-size=512m -e TEST_ARGS="--grep firefox.*68.*extension" -it browsers
...
  1) Browser: firefox
       Version: 68.0
         loads an extension:
     TimeoutError: /index.html did not open
Wait timed out after 8183ms

Please notice the same test works fine when using geckodriver@3.1.0.

What to do

Investigate why that would happen, maybe starting by spotting which exact version change makes the test fail, investigate those release notes, etc. If possible, fix the issue.